Infrared radiation is a subset of the electromagnetic spectrum and has a wavelength longer than that of visible light. Infrared radiation is invisible to humans and may be present in situations devoid of visible light. For example, a warm object, such as a person, will emit infrared radiation in a dark environment. Additionally, infrared radiation may penetrate objects that visible light is unable to penetrate because infrared radiation has a longer wavelength than visible light. As an example, infrared radiation created by a heat source, such as a fire, may penetrate a wall that is opaque to visible light.
Infrared imaging detectors, which are well known in the art, may be used to detect infrared radiation. Such imaging detectors may be used to create an image of a scene of interest by capturing and processing infrared radiation emitted from the scene. For example, an infrared imaging detector may be used as a night vision apparatus to display an image of a scene of interest at night by capturing infrared radiation emitted from the scene. As another example, an infrared imaging detector may be used to display an image of a scene of interest in foggy conditions or to detect an existence of a fire within a building's wall. Such uses of an infrared imaging detector are possible because infrared radiation may penetrate the fog and the wall.
Infrared imaging detectors generally contain an array of a plurality of photodiodes, which capture infrared radiation emitted from a scene of interest. Each photodiode is electrically biased such that it generates an electric current in response to electromagnetic energy of a certain range of wavelengths impinging the photodiode. However, in many applications, it is desirable that the photodiode generate an electric voltage corresponding to the electric current integrated over a period of time, as opposed to an instantaneous electric current, in response to infrared radiation impinging the photodiode. Consequently, each photodiode is generally electrically connected to a respective signal processing system, often referred to as unit cell, which integrates and converts electric current generated by the photodiode into an electric voltage signal. The electric voltage signal may then be read and processed by additional subsystems. For example, a unit cell may convert electric current generated by its respective photodiode into an electrical voltage signal, which is subsequently read, buffered, and amplified by additional subsystems. A representation of the buffered and amplified electric voltage signal may then be displayed on a screen wherein the image displayed on the screen reflects electromagnetic energy emitted from a scene of interest. Each individual photodiode and its corresponding unit cell may be referred to as a pixel.
As stated above, an infrared imaging detector generally includes a plurality of photodiodes, and each photodiode is electrically coupled to a respective unit cell. A plurality of unit cells are often integrated into a single integrated circuit commonly referred to as a read-out integrated circuit (“ROIC”). A ROIC is commonly physically and electrically coupled to an array of photodiodes, often by a plurality of microscopic indium bumps. A combination of a ROIC and array of photodiodes is commonly referred to as a focal plane array (“FPA”). Accordingly, a FPA includes a plurality of pixels.
A given photodiode is generally optimized to efficiently generate an electric current in response to impinging electromagnetic radiation having a certain range of wavelengths. For example, one type of photodiode might be optimized to detect “midwave” infrared radiation, which has a wavelength ranging from 2.5 to 5.0 microns, while another type of photodiode might be optimized to detect “longwave” infrared radiation, which has a wavelength ranging from 5.0 microns to 11 or 12 microns. The range of wavelengths that a particular photodiode is optimized to detect is a function of factors including the photodiode's constituent materials and the operating conditions (e.g. bias voltage) of the photodiode.
The following are examples of various types of photodiodes and the lengths of electromagnetic radiation that they may efficiently detect. Silicon photodiodes may detect electromagnetic radiation ranging from ultraviolet to near infrared and having corresponding wavelengths ranging from 400 to 1,000 nanometers (nm). Gallium-Arsenide (GaAs) and Germanium (Ge) photodiodes may detect electromagnetic radiation having a wavelength up to 1,800 nm. Indium-Gallium-Arsenide (InGaAs) photodiodes may detect electromagnetic radiation having a wavelength ranging from 0.8 to 2.6 micrometers (μm). Indium-Antimony (InSb) and Indium-Arsinide (InAs) photodiodes may detect electromagnetic radiation having a wavelength up to approximately 5 μm. Mercury-Cadmium-Teluride (HgCdTe) photodiodes may detect electromagnetic radiation having a wavelength up to 16 μm. HgCdTe photodiodes are commonly used in infrared imaging detectors because they can detect electromagnetic radiation well into the infrared radiation portion of the electromagnetic spectrum. In addition, HgCdTe photodiodes can be optimized to capture electromagnetic radiation of a desired wavelength by tuning their bandgaps through varying the relative composition of their constituent materials.
It is often desirable that a FPA be operable to detect infrared radiation or light having at least two different wavelengths or colors, such as midwave infrared radiation or light (“MWIR”) and longwave infrared radiation or light (“LWIR”). Such multicolor detection capability is desirable because different colors of infrared radiation may have unique useful properties. For example, LWIR has a greater flux density (quantity of photons per unit area per unit time) than MWIR; consequently, LWIR may be detected at a greater distance than MWIR. However, MWIR allows creation of images having a greater contrast than LWIR, therefore, MWIR allows for superior image recognition. Consequentially, an infrared imaging detector capable of detecting and processing both LWIR and MWIR will advantageously be able to simultaneously optimize long-range detection and image recognition.
As stated above, a photodiode is generally optimized to detect electromagnetic radiation having a certain range of wavelengths. Therefore, FPAs generally have a plurality of photodiodes optimized for each wavelength of electromagnetic radiation that the FPA is designed to detect. For example, if a FPA is designed to detect both LWIR and MWIR, the FPA will include a plurality of LWIR optimized photodiodes and MWIR optimized photodiodes.
Prior art multi-color infrared imaging detectors either employed two or more FPAs, wherein each FPA was optimized to detect a single color, or employed a two dimensional FPA as a one dimensional scanning array, wherein the other dimension was used to detect a plurality of colors. Recent advances have created simple, multi-color ROICs intended for use with photodiode arrays sensitive to two or more wavelengths or colors of infrared radiation. Such multi-color ROICs generally contain a plurality of essentially identical unit cells that are not optimized for their respective photodiodes. Consequently, prior art ROICs do not offer optimal signal processing capability when coupled to an array of photodiodes optimized for two or more colors of infrared radiation. Hence, there is a need for a ROIC that includes unit cells that may be optimized to detect infrared radiation having at least two colors.